Aggression and depression in childhood have captured a great deal of empirical attention in recent years. However, despite evidence for considerable overlap between these two serious mental health problems, relatively few studies have addressed issues of co-morbidity. Particularly lacking is research on: (1) the mechanisms by which childhood aggression may result in depressive -symptoms (e.g., relational vulnerability involving relatively high levels of aggression, relatively high levels of peer-related stress, and relationally-biased social information processing); (2) forms of aggression that are typical of girls (e.g., "relational aggression") rather than a singular focus on forms of aggression most characteristic of boys (e.g., physical aggression); and (3) gender differences in childhood risk factors for depressive symptoms that might be related to gender differences in depressive symptoms during adolescence (e.g., relational vulnerability). These issues will be addressed in the present study through the longitudinal study of a large sample of children (n=600), initially recruited and assessed during the beginning of their fourth-grade year of elementary school (Time 1), and then reassessed at the end of fourth grade (Time 2) and the beginning of fifth grade (Time 3).